The invention relates to a device for correcting the wavelength dependence in diffraction-based optical systems in which a filtering of certain diffraction orders is provided, said device comprising at least one diffractive optical light modulator having controllable structures and at least one light source for illuminating the light modulator, where corresponding diffraction orders are generated which exhibit a wavelength-dependent lateral chromatic offset D, related to the surface normal of the light modulator, of the position of their different extents BOR, BOG, BOB in a filtering plane which is defined by the focal length of a subsequent optical system. The invention relates to both amplitude-modulating and phase-modulating light modulators and does not depend on the technological basis of the modulators. It can be realised with liquid crystal modulators as well as with modulators which are based on micro electro-mechanical systems (MEMS), acousto-optic or other modulators.
Spatial light modulators (SLM), for example realised on the basis of liquid crystals, are areal optical elements which reflect or transmit visible light and whose optical properties can be temporarily modified by applying an electric field. The electric field can be controlled discretely for small structures, also referred to as pixels, which allows the optical transparency properties of the light modulator to be modified pixel-wise but finely enough for many applications. Advantage is taken of this possibility for example in order to modify by way of amplitude or phase modulation an incident wave front during its passage though the light modulator such that, at the observer's distance, it resembles a wave front which is emitted by a real object. If the light modulator is controlled accordingly, for example a holographic reconstruction of a spatial object becomes possible without the need for this object to be actually present at the time of its observation.
Due to the discrete pixel structure of the light modulators, the diffraction pattern is repeated periodically in consecutive diffraction orders, while its intensity decreases as the ordinal number rises. It is therefore necessary for example when holographically reconstructing objects to filter a certain diffraction order—usually the first one—out of the periodic diffraction spectrum and to suppress the other diffraction orders. One problem therein is that the diffraction orders exhibit different orientations and angular extents for different wavelengths, which causes a lateral offset and a different width of the diffraction patterns in a defined filtering plane. This is why during mechanical filtering, e.g. with the help of an aperture with a defined diameter, there may be a loss of information and/or parasitic cross-talking in a certain position, in particular when reconstructing colour objects.
Document DE 10 2005 023 743 describes a method for filtering diffraction orders, where with the help of a given aperture in a particular plane—the filtering plane—irrelevant information is cut off.
In document US 2006033972 A1, that problem is solved by disposing the light sources of the different colours, LQR, LQG, LQB, which illuminate the light modulator, at such mutual distances that the diffraction orders for the three colours overlap at the same position after diffraction at the structures of the light modulator. However, this is not possible if the individual colours originate in the same light source, i.e. if a white light source is used, or if the light sources of the different colours are disposed at fixed mutual distances, e.g. as is the case when using a colour display panel as a light source.
Summarising, light modulators are diffractive optical elements whose chromatic dispersion is caused by the wavelength dependence of the diffraction angle, which cannot be avoided. In addition to diffractive optical elements (DOE), there are also refractive optical elements (ROE), where also in refractive optical elements a chromatic dispersion occurs, which means that the refraction angle varies as the wavelength of the incident light changes. Refractive dispersion is caused by the dependence of the refractive index on the wavelength.
The refractive dispersion of a lens is described for example by E. Mutter in the document “Kompendium der Photographie”, vol. I, Verlag für Radio-Foto-Kinotechnik GmbH, Berlin-Borsigwalde, 1958, pp. 270-271, where each lens functions like a double prism, and the rays of shorter wavelength, i.e. the blue rays, intersect closer to the lens than the rays of longer wavelength, i.e. the red rays, due to the stronger refraction of the former. This is why there is a number of focal points one behind another in a row for the different spectral rays. In contrast to the asymmetry of a prism, a lens is a symmetrical optical element.
Instead of sharp image points, the chromatic refraction causes coloured circles of dispersion to appear around the image points so as to give them a certain kind of blur. The refraction of a lens made of glass showing a certain chromatic dispersion can be limited with the help of the refraction of another lens made of glass showing a different chromatic dispersion. A thus corrected lens, which comprises a low-refraction and high-dispersion focussing lens made of crown glass and a high-refraction and low-dispersion diverging lens made of flint glass, is also referred to as an achromatic lens. The achromatic lens unites two colours of the spectrum, namely the Fraunhofer lines C and F. For higher-quality photographic recordings, a correction of the refraction is performed by uniting three wavelengths.
The chromatic dispersion of a refractive optical system can be specified by its Abbe number V:V=(nd−1)/(nF−nC)  (I)
where nd is the refractive index of the glass material at the wavelength of neutral helium at 587.6 nm (yellow), and nF and nC are the refractive indices at the wavelengths of neutral hydrogen at 656.3 nm (red) and 486.1 nm (blue). The larger the Abbe number V, the greater the dispersion of the glass material.
The extents and the main directions of the diffraction orders are proportional to the wavelength in the filtering plane, which typically forms the focal plane of the optical system. Therein, a mechanical filter in the form of an aperture mask where e.g. the given diffraction order of the blue wavelength is considered only, which is suitable for the blue light, will cut off part of the red information of the given diffraction order of the red wavelength. There is thus neither a suitable width nor a suitable position of the aperture in the filtering plane.
This way the filtering can cause a great loss of information of a certain colour, in this example the red colour, or an inadequate filtering which does not contain all appropriate information of another colour, while parasitic light is let past, which is known as cross-talking.
One problem is that the diffraction orders have a spatial extent and an orientation which is extremely dependent on the wavelength, so that they exhibit only small overlapping sections, so that they cause a noticeably perceivable loss of information e.g. when holographically visualising objects.
It is therefore the object of the present invention to provide a device for correcting the wavelength dependence in diffraction-based optical systems, said device being designed such that the diffraction orders of the same ordinal number of light of different wavelengths, which is diffracted in the light modulator, overlap sufficiently both as regards their direction and their extent in a given filtering plane.
The object is solved with the help of the features of claim No. 1.
The device for correcting the wavelength dependence in diffraction-based optical systems, in which a filtering of certain diffraction orders is provided, said device comprising at least one diffractive optical light modulator having controllable structures and at least one light source for illuminating the light modulator, where corresponding diffraction orders are generated which exhibit a wavelength-dependent lateral chromatic offset D, related to the surface normal of the light modulator, of the position of their different extents BOR, BOG, BOB in a filtering plane which is defined by the focal length of a subsequent focussing optical system where according to the characterising clause of claim No. 1 the diffractive light modulator is followed by the refractive focussing optical system whose chromatic properties regarding the wavelength-dependent diffraction orders of the same ordinal number are adapted to the chromatic diffraction of the same diffraction orders of the same ordinal number of the light modulator, where in a given filtering plane after the focussing optical system the diffraction orders (BOR, BOG, BOB) of the same ordinal number and of different wavelengths (red, green, blue) overlap as concentrically as possible.
The amount of the refractive dispersion of the focussing optical system can be identical to the amount of the diffractive dispersion of the light modulator, where refractive dispersion and diffractive dispersion act in opposing directions and compensate each other substantially.
In the filtering plane there can be disposed a filtering aperture which only lets pass selected diffraction orders of the same ordinal number and of different wavelengths (red, green, blue).
The focussing optical system can comprise multiple components, preferably at least two lenses.
In at least one lens of the focussing optical system, the refractive indices for the wavelengths of red, green and blue depend on the Abbe number V according to the equationV=(nd−1)/(nF−nC)  (I)and the diffraction patterns of the wavelengths of red, green and blue of the selected diffraction orders of the same ordinal number are minimised as regards the refractive index nd of the yellow wavelength at a distance d to the filtering plane.
The lenses can form a doublet lens, where one lens exhibits a given Abbe number V1 and the other lens exhibits an Abbe number V2 which is adapted to the Abbe number V1 of the former lens.
The doublet lens can be a doublet lens with like geometric parameters, where the doublet lens can for example comprise two plano-convex lenses which are disposed such that their plane faces are parallel and facing each other.
The refractive indices nd, nF, nC of the Abbe number V2 of the second lens of the doublet lens can be determined based on a given value of the Abbe number V for the largely concentric overlapping of the diffraction patterns of the corresponding wavelengths of red, green and blue in the focal plane for the yellow wavelength.
It is of major importance that the remaining parameters of the focussing optical system, such as the reference focal lengths and main planes, do not change.
It can be possible to select the glass material or the glass materials of the given component—preferably the second lens—of the focussing optical system based on the given and determined Abbe numbers V and the corresponding refractive indices nd, nF, nC.
The light source can be a single white light source, which contains the three wavelengths of red, green and blue.
The light source can alternatively be a light source unit with the light sources of the individual colours LQR, LQG, LQB with the wavelengths of blue, green, red, which are optionally disposed at the same position or at various positions in a plane which is preferably arranged at a right angle to the surface normal.
The light modulator can have an optically active layer, preferably in the form of a plane birefringent layer, which contains liquid crystals, and whose refractive index ellipsoid is controllable by applying an electric field to the structures in the form of pixels.
The light modulator can also comprise controllable electromechanical structures—MEMS—with diffractive optical properties or be realised based on other technologies, e.g. acousto-optic.